Rapid actions of 1α,25‐dihydroxyvitamin D3 on Ca2+ and phospholipids in isolated rat liver nuclei

Abstract

The effects of 1α,25-dihydroxyvitamin D₃ (1α,25-(OH)₂D₃) on Ca²⁺ levels and phospholipid metabolism were studied in isolated nuclei prepared from rat liver. Nuclear Ca²⁺ concentration was estimated with the fluorescent indicator Fura 2. In agreement with previous reports, ATP (1 mM) produced a rapid increase in nuclear Ca²⁺ from 188 ±25 to 593 ±121 nM. Exposure to 1α,25-(OH)₂D₃ (20 nM) also produced a rapid increase in nuclear Ca²⁺ to 402±71 nM. The 1β epimer of lα,25-(OH)₂D₃ had no effect. Nuclear phosphatidylinositol was labeled by incubation with [γ-³²p]ATP for 3 h. 1α,25-(OH)₂D₃ produced a two-fold increase in [³²P]lysophosphatidylinositol (LPI) within 5 min from 44 ± 11 to 87 ± 19 cpm/2.5 × 10⁷ nuclei. 1β,25-(OH)₂D₃ had no effect on [su32P]LPI production. Exposure of nuclei to exogenous LPI (15μM) produced an instantaneous increase in nuclear Ca²⁺ to 372±81 nM, comparable to ATP and lα,25-(OH)₂D₃. The rapid effects of 1α,25-(OH)₂D₃ on phospholipid metabolism and Ca²⁺ in isolated nuclei suggest that the steroid may exert effects distinct from the well-characterized receptor-mediated changes in gene expression.